<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <meta http-equiv="X-UA-Compatible" content="ie=edge">
    <title>Document</title>
    <!-- id选择器 -->
    <!-- 根据id选择标签 以#开头 元素的id名称不能重复 -->
    <style>
         #mydiv{
             color: red
         }

    </style>
</head>
<body>
<div id=mydiv>test</div>
<div>test</div>
</body>
</html>